Each year the U.S. generates approximately 290 million scrap tires. About 12 million scrap tires are converted into ground tire rubber for modifying asphalt cements. The utilization of scrap tire rubber in asphalt started in the mid-1960's when ground tire rubber was placed in asphalt surface treatments, such as chip seal applications.
In terms of environmental concerns, the disposal of scrap tires is a major waste management issue. While these environmental concerns are important, there are various challenges associated with using ground tire rubber.
For example, with respect to hot applied chip seal, the inconsistent dissolution of ground tire rubber in asphalt can result in blockages to hot asphalt spray systems that apply the ground tire rubber/asphalt mixture. With respect to asphalt emulsions, the inconsistent dissolution of ground tire rubber in asphalt can interfere with the emulsification of asphalt in water due to the interference of the discrete rubber particles in the asphalt water inversion process. With respect to asphalt cutback, inconsistent dissolution of ground tire rubber in asphalt can result in blockages in cold asphalt spray systems used to apply the GTR solvent cut back asphalt to the road surface when used a gravel chip seal. With respect to tack coats, inconsistent dissolution of ground tire rubber in asphalt can interfere with even distribution and blockages in the spray applicator that may ultimately result in inferior pavement systems due to improper layers adhesion. With respect to pavement membranes, inconsistent dissolution of ground tire rubber in asphalt can interfere with even distribution and blockages in the spray applicator that may ultimately result in inferior pavement systems due to improper adhesion between the reinforcements and the pavement layers. With respect to pavement joint and caulking fillers, inconsistent dissolution of ground tire rubber in asphalt used in caulking formulations can interfere with even distribution and blockages in various applicators that may ultimately result in inferior sealing for pavement joints shortened road longevity. With respect to driveway sealers, inconsistent dissolution of ground tire rubber in asphalt used in driveway seal formulations can interfere with even distribution and sealing of the drive surface resulting in reduced protection.
The process of combining rubber with asphalt is described in various patents and patent publications.
For example, in U.S. Pat. No. 5,342,866, Trumbore teaches an elastomeric-asphalt composition which does not phase separate and which is compatible at high temperatures. In a preferred embodiment, the elastomeric materials are SBS and SIS block copolymers. Producing the asphalt composition includes preheating the asphalt to a temperature of at least about 350° F. Additionally, Trumbore teaches heating the mixture to a temperature of from about 400° to about 475° F. while maintaining the pressure in the converter under about 2 psig.
Nielsen in U.S. Pat. No. 4,068,023 teaches one of the earliest examples showing the incorporation of reclaimed rubber into asphalt for paving using elevated temperatures up to 450° F. with an admixture of aromatic oils.
In European Patent Application EP1877493 A2, Martin teaches a modified asphalt binder composition that includes about 40% by weight to about 98.9% by weight asphalt binder material, about 0.5% by weight to about 25% by weight crumb rubber, about 0.5% by weight to about 30% by weight of at least one synthetic polymer, and about 0.05% by weight to about 5% by weight of at least one acid. Martin also teaches a method for making a modified asphalt binder composition that includes providing neat asphalt, heating the neat asphalt to a temperature of between about 120° C. (248° F.) and about 200° C. (392° F.), adding a first modifying ingredient to the neat asphalt, mixing the asphalt and the first modifying ingredient with one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours, adding a second modifying ingredient to the modified asphalt binder, mixing the second modifying ingredient and the modified asphalt binder in one of a high shear mixer or a low shear mixer for a period of between about 5 minutes and about 10 hours, adding a third modifying ingredient to the modified binder material, and agitating the third modifying ingredient and the modified binder material in one of a low shear mixer or a high shear mixer for a period of between about 5 minutes and about 48 hours.
In U.S. Pat. No. 5,492,561 Flanigan teaches incorporating the whole tire rubber into the asphalt medium by simulating a “boiling action” in the asphalt medium which allows the tire rubber to be absorbed into the asphalt medium at about 500° F. Flanigan states that below 485° F.-490° F. provides insufficient blending, while above 510° F. the temperature is too close to the flash point of the liquid. According to Flanigan, a temperature of about 500° F. is the safest temperature to use that is high enough to provide full incorporation of whole tire rubber granules into the asphalt medium but not so high that the process becomes unsafe. The Flannigan process produces a dissolved ground tire rubber after extended blending of 5-10 hours at 500° F., which degrades the asphalt blend by initiating an extended oxidation reaction that affects the quality of the asphalt.